1. Field of the Invention
This invention relates to a method and apparatus for the real-time, non-destructive evaluation of adhesion bonds using stress-wave emission techniques, and more particularly, to method and apparatus for evaluating an adhesion bond by measuring the stress-wave energy emitted during the bonding cycle and comparing the measured energy with a predetermined substantially linear relationship between the emitted stress-wave energy and the strength of the adhesion bond to determine the strength of the bond.
2. Description of the Prior Art
The ability to evaluate the strength of an adhesion bond has in recent years become increasingly important with the increased use of thermocompression, ultrasonic and other bonding techniques. The most common method used for determining the strength of an adhesion bond is the standard peel test. Such test, however, destroys the bond and is, therefore, not a very desirable test, since it can only be used on a spot-check basis.
A method for non-destructively indicating the quality of an adhesion bond formed by vibratory bonding techniques is disclosed in U.S. Pat. No. 3,302,277, issued to D. H. Pruden et al. on Feb. 7, 1967. There, a test current is passed through an electrical component being bonded to an element and a change in the voltage drop across the bond area measured. The measured voltage drop is then used to indicate the quality of the bond, and alternatively to control the amount of vibratory energy introduced into the bonding zone.
Stress-wave emission detection and measuring techniques have been used for detecting low amplitude, short duration, and fast rise time pulses emitted by cracks forming in brittle materials when the material is subjected to a load as, for example, during a bonding operation or pressure test. In this regard, see, for example, a report dated May 19, 1972, entitled, "Investigation of Acoustic Emission from Ceramic-Materials," by G. A. Alers, No. SC 513.6FR, published by North American Rockwell Science Center.
U.S. Pat. No. 3,965,726, which issued on June 29, 1976 to the present inventor and is assigned to the same assignee, evaluates spot welds by measuring the stress waves emitted during various time periods of the welding cycle, each time period corresponding to a different aspect of the weld cycle such as, for example, the solid-to-liquid phase transformation and the liquid-to-solid phase transformation in the weld area since each of these elements, and others, can affect the quality and/or the extent of a weld. This patent also discloses that a relatively linear relationship exists between the net resultant stress-wave energy value which is the difference in energy between the solid-to-liquid and the liquid-to-solid phase transformations of a spot weld. The problem still remains of providing method and apparatus which is inexpensive and which non-destructively determines the strength of an adhesion bond in real time.